The development of high-efficiency power supplies in combination with a requirement of higher power density is a continuing goal in the field of power electronics. A switched-mode power converter is a frequently employed component of a power supply that converts an input voltage waveform into a specified output voltage waveform. There are several types of switched-mode power converters including, for instance, an asymmetrical half-bridge power converter.
A conventional asymmetrical half-bridge power converter includes two power switches coupled to a controller, at least one isolation transformer, a voltage balancing capacitor, a rectifier and a filter. The asymmetrical half-bridge power converter generally operates as follows. The first and second power switches conduct current in a complimentary manner, with generally unequal duty cycles, to convert an input DC voltage into an AC voltage to be applied across the isolation transformer. Any DC component of the voltage applied to a primary winding of the isolation transformer is blocked by the voltage balancing capacitor coupled in series with the primary winding of the isolation transformer. The rectifier then rectifies a secondary voltage from the isolation transformer and the filter smooths and filters the rectified voltage to develop an output voltage for delivery to a load. The controller monitors the output voltage of the asymmetrical half-bridge power converter and adjusts the duty cycle of the power switches to ultimately control the output voltage. The output voltage may be maintained at a relatively constant level despite relative fluctuations in the input voltage and the load.
The asymmetrical half-bridge power converter is a well known power circuit topology that, when operating in a flyback mode, may be capable of zero voltage switching (ZVS) operation. A high magnetizing current, usually exceeding twice the load current, however, may be required to attain ZVS operation. Further, one or more of the power switches may be subject to current spikes induced therein by the reverse recovery of the rectifier.
In U.S. Pat. No. 5,402,329, entitled, "Zero Voltage Switching Pulse Width Modulated Power Converters, incorporated herein by reference, Wittenbreder suggests placing an inductor in series with the primary winding of the isolation transformer. One of the ZVS transitions is driven by a combination of the magnetic energy stored in the transformer and the magnetizing energy stored in the inductor. The other ZVS transition is driven by energy stored in the inductor. While the series inductance may allow for ZVS operation with lower magnetizing currents, the inductor may cause spurious voltage spikes across the rectifier during reverse recovery.
Accordingly, what is needed in the art is a circuit, employable with a variety of power converter topologies, that reduces voltage spikes across a rectifier of the power converter during a reverse recovery phenomenon and advantageously recovers energy from the inductor to increase an efficiency of the power converter.